This invention relates to a solid-oxide electrolyte structure useful in an electrochemical energy converter for fuel to electricity conversion under a fuel cell (electric generator) operating mode or electricity to fuel conversion under an electrolyzer (fuel synthesizer) operating mode and to a method for making the same. The converter is capable of yielding high efficiencies and depends only on the relation between free energy and enthalpy of the electrochemical reaction and is not limited by Carnot-cycle considerations.
A key component in an electrochemical energy converter is the electrolyte on which an oxidizer electrode and a fuel electrode are applied. The electrolyte must be an ionic conductor with acceptably low resistance and must be capable of transporting an ionic reaction species from one electrode-electrolyte interface to the opposite electrode-electrolyte interface under the operating conditions for the converter. It is well known that zirconia stabilized with oxides such as magnesia, calcia or yttria satisfies the requirements when operating at high temperature (about 1800.degree. F. or about 1000.degree. C.). This material utilizes oxygen ions to carry electrical current. The electrolyte should be electronically non-conducting in order not to short-circuit the converter. On the other hand, the electrode must be a good electron conductor. Interaction of the reacting gas, electrode and electrolyte occurs at the electrode-electrolyte interface which requires that the electrodes be sufficiently porous to admit the reacting gas and to permit exit of product gases.
Prior to the present invention, zirconia electrolyte as disclosed by U.S. Pat. No. 3,460,991 has been shaped in tubular configuration. It has proven to be mechanically delicate, prone to fraction under thermal cycling and has low volumetric power density. The design disclosed by U.S. Pat. No. 3,554,808 adopts planar configuration for the fuel cell batteries. However, the in-situ sintering of components in constructing a stack of these planar zirconia electrolytes has proven difficult.
An approach to overcome these problems by forming the electrolyte in free-standing plates was disclosed by Hsu, et al, in the Proccedings of the 11th Intersociety Energy Conversion Engineering Conference, in the article entitled "Electrochemical Power and Hydrogen Generation from High Temperature Electrolytic Cells". This attempt has not been successful primarily due to the fact that the surface of the electrolyte has a corrugated shape which was utilized to permit passage of fuel and oxidizer. This electrolyte configuration has proven to be very difficult to fabricate since it has a tendency to fracture while being shaped. In addition, this approach does not address the synergetic needs for electric contacts and gas seals between the adjacent plates in the stack assembly to achieve low loss, high operating efficiency. Furthermore, this design does not permit the use of air or other mixture of gases containing oxygen as no means are provided for removal of the product gas at the oxidizer side.
It would be desirable to provide a solid oxide electrochemical converter which is feasible to construct, rugged in operation and handling, flexible in fuel or oxidizer selections and which provides low electrical or fuel losses.